The following references disclose various aspects related to, inter alia, optical frequency combs, tunable laser sources, high-Q optical resonators, and second harmonic generation. Some of those disclosed aspects may be employed in examples of inventive apparatus or methods disclosed or claimed herein. Each of the following references is incorporated by reference as if fully set forth herein:    [1] P. Del Haye, A. Schliesser, O. Arcizet, T. Wilken, R. Holzwarth and T. J. Kippenberg, “Optical frequency comb generation from a monolithic microresonator”, Nature 450, 1214 (2007);    [2] M. J. R. Heck, J. F. Bauters, M. L. Davenport, J. K. Doylend, S. Jain, G. Kurczveil, S. Srinivasan, Y. Tang and J. E. Bowers. “Hybrid Silicon Photonic Integrated Circuit Technology”, IEEE J. Sel. Topics Quant. Electron. 19, 6100117 (2013);    [3] J. C. Hulme, J. K. Doylend, and J. E. Bowers, “Widely tunable Vernier ring laser on hybrid silicon”, Opt. Express 21, 19718-19722 (2013);    [4] T. Komljenovic, et al., “Heterogeneous Silicon Photonic Integrated Circuits”, J. Lightwave Technol. 34, 20-35 (2016);    [5] S. Srinivasan, M. Davenport, T. Komljenovic, J. Hulme, D. T. Spencer, and J. E. Bowers, “Coupled-Ring-Resonator-Mirror-Based Heterogeneous III-V Silicon Tunable Laser”, IEEE Photon. J. 7, 2700908 (2015);    [6] T. Komljenovic, S. Srinivasan, E. Norberg, M. Davenport, G. Fish, and J. E. Bowers, “Widely Tunable Narrow-Linewidth Monolithically Integrated External-Cavity Semiconductor Lasers”, IEEE J. Sel. Topics Quantum Electron. 21, 1501909 (2015);    [7] J. Li, H. Lee, T. Chen, and K. J. Vahala, “Low-Pump-Power, Low-Phase-Noise, and Microwave to Millimeter-Wave Repetition Rate Operation in Microcombs”, Phys. Rev. Lett. 109, 233901 (2012);    [8] H. Lee, T. Chen, J. Li, K. Y. Yang, S. Jeon, O. Painter and K. J. Vahala, “Chemically etched ultrahigh-Q wedge-resonator on a silicon chip,” Nature Photon. 6, 369-373 (2012);    [9] Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute Optical Frequency Measurement of the Cesium D1 Line with a Mode-Locked Laser”, Phys. Rev. Lett. 82, 3568 (1999);    [10] M. H. P. Pfeiffer, A. Kordts, V. Brasch, M. Zervas, M. Geiselmann, J. D. Jost, and Tobias J. Kippenberg, “Photonic Damascene process for integrated high-Q microresonator based nonlinear photonics”, Optica 3, 20 (2016);    [11] Q. Li, T. C. Briles, D. A. Westly, J. R. Stone, B. R. Ilic, S. A. Diddams, S. B. Papp, and K. Srinivasan, “Octave-spanning microcavity Kerr frequency combs with harmonic dispersive-wave emission on a silicon chip”, Frontiers in Optics/Laser Science, Paper FW6C.5 (2015);    [12] N. Volet, A. Spott, E. J. Stanton, M. L. Davenport, J. Peters, J. Meyer, and J. E. Bowers, “Semiconductor optical amplifiers at 2.0-μm wavelength heterogeneously integrated on silicon”, in Conference on Lasers and Electro-Optics SM4G.4 (2016);    [13] L. Chang, Y. Li, N. Volet, L. Wang, J. Peters, and J. E. Bowers, “Thin film wavelength converters for photonic integrated circuits”; Optica 3, 531 (2016);    [14] Z. Li, Y. Fu, M. Piels, H. Pan, A. Beling, J. E. Bowers, and J. C. Campbell, “High-power high-linearity flip-chip bonded modified unitraveling carrier photodiode”, Opt. Express 19, B385-6390 (2011);    [15] M. L. Davenport, S. Skendžić, N. Volet, J. C. Hulme, M. J. R. Heck, and J. E. Bowers, “Heterogeneous Silicon/III-V Semiconductor Optical Amplifiers”, IEEE J. Sel. Topics Quantum Electron., 22, 3100111 (2016);    [16] A. Spott, M. Davenport, J. Peters, J. Bovington, M. J. R. Heck, E. J. Stanton, I. Vurgaftman, J. Meyer, and J. Bowers, “Heterogeneously integrated 2.0 μm CW hybrid silicon lasers at room temperature”, Opt. Lett. 40, 1480-1483 (2015);    [17] M. Piels, J. F. Bauters, M. L. Davenport, M. J. R. Heck, and J. E. Bowers, “Low-Loss Silicon Nitride AWG Demultiplexer Heterogeneously Integrated With Hybrid III-V/Silicon Photodetectors”, J. Lightwave Technol. 32, 817-823 (2014);    [18] Yang, K. Y., Oh, D. Y., Lee, S. H., Yang, Q.-F., Yi, X. & Vahala, K. Integrated Ultra-High-Q Optical Resonator. arXiv preprint arXiv:1702.05076 (2017);    [19] U.S. Pat. No. 8,818,146 entitled “Silica-on-silicon waveguides and related fabrication methods” issued Aug. 26, 2014 to Vahala et al;    [20] U.S. Pat. No. 9,293,887 entitled “Chip-based laser resonator device for highly coherent laser generation” issued Mar. 22, 2016 to Li et al; and    [21] U.S. non-provisional application Ser. No. 15/587,897 entitled “High-Q optical resonator with monolithically integrated waveguide” filed May 5, 2017 in the names of Vahala et al.